nclosed
within sheet piling, to prevent settlement and the infiltration of
water under pressure below the dock. Keel blocks are laid along the
centre line of the dock, for the keel of the vessel to rest on when
the water is pumped out; and the vessel is further supported on each
side by timber shores supported on the steps or "altars" of the side
walls, which are lined with granite or other hard stone, or blue
bricks, or, when constructed of concrete, with a facing of stronger
concrete, to enable these altars to withstand the wear and shocks to
which they are subjected. Steps and slides are provided at convenient
places at the sides to give access for men and materials to the bottom
of the dock; and culverts and drains lead the water to pumps for
removing the water from the dock when the entrance has been closed,
and to keep it dry whilst a vessel is under repair. Culverts in the
side walls of the entrance enable water to be admitted for filling the
dock to let the vessel out. Graving docks are generally closed by ship
caissons; but where they open direct on to a tidal river, and there is
some exposure, gates are adopted, or sometimes sliding caissons.

The dimensions of graving docks vary considerably with the nature of
the trade and the date of construction; and sometimes an intermediate
entrance is provided to accommodate two smaller vessels. The sizes of
some of the largest graving docks are as follows: Liverpool, Canada
dock, 925-1/2 ft. long, 94 ft. width of entrance, and 29 ft. depth at
the ordinary water-level in the dock; Southampton, 851-3/4 ft. by 90
ft., and 29-1/2 ft. depth at high-water neaps (figs. 26 and 27);
Tilbury, 875 ft. by 70 ft. by 31-1/2 ft.; and Glasgow, 880 ft. by 80
ft. by 26-1/2 ft.

_Floating Dry Docks._--Where there is no site available for a graving
dock, or the ground is very treacherous, floating dry docks, built
originally of wood, but more recently of iron or steel, have
occasionally been resorted to. The first Bermuda dock towed across the
Atlantic in 1869, and the new dock launched in 1902, 545 ft. by 100
ft., are notable examples. Water is admitted into the pontoon at the
bottom to sink the dock sufficiently to admit a vessel at its open
end; and then the water is pumped out of compartments in the pontoon
till the vessel is raised out of water. It is only necessary to find a
sheltered site, with a sufficient de